The subject of the invention is a method and a circuit for the purpose, for instance on sound films, of decoding four channel signals, i.e. the right, left, centre and background, which are coded in a matrix and are available in the form of a two-channel signal, by the application of amplifiers for the right and left channels respectively, and a summation applifier forming the sum of the right and left stereo channels, for the centre channel, and a differential amplifier forming the difference between the right and left stereo channels, for the background channel, whereby in each channel an automatic control of the amplification will take place by means of an output amplifier in the output stage in question.
On sound films for stereo reproduction of the sound the two optical sound tracks are placed between the frame field and the perforation, which ensures the synchronised transport of the films. On these sound tracks the right and left channels are recorded directly, and on top of these channels a mono signal such as the dialogue is recorded with equal amounts of information on both tracks. Effect sounds, such as pistol shots or screeching car tyres, are always recorded in stereo. FIG. 1 shows a representation of a known system for the type referred to above, such as it is known. To obtain a greater stereo effect, a controllable amplifier has been inserted into each of the four channels. This one increases the amplification when the voltage over the capacitor C increases. In some cases D and C are replaced by a phase detector, which measures the phase difference between the various channels and adjusts accordingly. However, these systems all have the drawback that noise impulses, for instance in the right channel, which--on a sound film--is most approximate to the perforation of the film, and which is thus most exposed to scratches and dirt, will cause an upwards adjustment of the right channel, whereby the centre channel will be adjusted half as much upwards, and an upwards adjustment of the background channel, as this one is constantly affected by the difference between the right and left channels. This will cause a total displacement of the stero picture and is thus an illusion-spoiling drawback.
A circuit for the production of three channel signals is known from DE patent specification No. 25.51.326, in which the centre channel is adjusted out of phase of the adjustment of the stereo channels right and left, and in which delay devices in the stereo channels are to ensure than the sound from the stereo channels does not reach the listener before the sound from the centre channel reaches him. Such an out of phase adjustment is inexpedient as, in case of small or no amplitudes on the sound tracks, the amplification will be increased to maximum either in the centre channel, because the stereo channels are adjusted totally down, or in the stereo channels, because the centre channel is adjusted totally down. This will obviously give a deteriorated signal-to-noise-ratio.